Projectors are used for business, for diagnostic purposes, for entertainment and for other purposes to project images from an image source onto a screen or the like for viewing. Many different types of projectors are known, some examples being, slide projectors, movie projectors, overhead projectors, and so forth. In some projectors an image which is fixed on a film, slide, or the like, is projected onto a viewing screen. In another type of projector an image is developed in a medium, such as a liquid crystal device, and the image is projected onto a viewing screen; sometimes these are referred to as liquid crystal projectors.
One example of liquid crystal projector uses a twisted nematic liquid crystal cell to modulate light from a light source to produce an image for projection. In such a projector polarized light is selectively transmitted or blocked by the cooperative relation between a twisted nematic liquid crystal cell and an optical polarizer. A disadvantage to such a projector is the lost efficiency due to the light blocking effect of a polarizer used to obtain polarized light. For example, a polarizer ordinarily blocks about 50% or more of the unpolarized incident light and, therefore, the maximum efficiency of transmission of light to bright areas of a projected image would be limited to 50% or less of the incident light intensity/brightness.
Another example of liquid crystal projector uses the principle of selective scattering or transmitting of light by a liquid crystal device in cooperation with an optical aperture and stop to provide an image for projection. In such a projector the liquid crystal device sometimes is referred to as NCAP, PDLC, LCPC and possibly other names. One example of such a projector is disclosed in U.S. Pat. No. 4,613,207. Several examples of liquid crystal materials or devices useful in such a projector are disclosed in U.S. Pat. Nos. 4,435,047, 4,688,900, and 4,606,611. For brevity, such materials and devices which are operable to scatter or to transmit light collectively may be referred to below as NCAP materials or NCAP devices. Also, for brevity, such other type of liquid crystal projector collectively may be referred to below as an NCAP based LCD projector ("LCD" being a conventional shorthand for liquid crystal display or liquid crystal device) or more simply as NCAP projector.
NCAP based LCD projectors which use the switchable light scattering properties of NCAP to modulate light have been demonstrated for example, as is disclosed in U.S. Pat. No. 4,613,207. In these projectors the light source is focused onto a small aperture located in a plate or mask between the projection lens and the NCAP display device. An image of the projector lamp filament (or arc, if applicable) is formed at the plane of this aperture. The aperture may be just slightly larger in diameter than the image of the lamp filament. When the NCAP device is in a clear (non-scattering) state, substantially all of the light passes through the aperture and is captured by the projection lens. Such transmitted light sometimes is referred to as specular light or specularly transmitted light. Light scattered by the NCAP device (sometimes referred to as scattered light) is redirected away from the aperture and is blocked or stopped by the plate in which the aperture is located. In such a projector system some of the scattered light may also impinge on and pass through the aperture; this light leakage can reduce contrast of the output. It would be desirable to improve contrast.
An alternative embodiment of the NCAP projector disclosed in the above '207 patent uses Schlieren optics that has some advantages over the above-described projector. Among these advantages is an improved dark field, which leads to higher contrast. One reason for such contrast improvement is improved discrimination between the light which is scattered by the NCAP device and the light which is transmitted by that device (specular light); the scattered light is collected and used in the output image, but the transmitted light (specular light) is blocked by a mask or stop in the light path and, therefore, does not reduce contrast.